In certain situations, it may be necessary to limit the communications sent to a given appliance when the latter is in a loading state such that it cannot cope with the various demands without degrading its processing times to a point which becomes prejudicial to the quality of the service rendered. The implementation of such a limitation is commonly called “call spacing” or “communication spacing”.
There exist today various solutions in which a congested appliance can dynamically control the implementation of a communication spacing procedure.
For example, in the case of switched telephone networks (STNs) and integrated services digital networks (ISDNs), provision is made for the possibility of a switch being able to indicate its loading state to other switches by means of messages exchanged at the message transport sub-system (MTSS) level so long as the congestion lasts. The receipt of messages of this type by a switch brings about a staggered reduction in the traffic sent to the congested switch until the receipt of these MTSS messages disappears.
Provision is also made to be able to integrate into these communications between switches a parameter indicating a loading level, for example, a high or low level.
The INAP (Intelligent Network Application Part) protocol, implemented in intelligent-network architectures, provides for interaction between two particular appliances ensuring respectively the functions of service access switch and services control point. A procedure allows the service control points to ask the service access switches to space out their triggering requests.
In another protocol termed “Megaco/H.248”, provision is made for a control message sent from a media gateway to a controller so that the controller spaces out the requisitions sent to a congested gateway.
Finally, in the so-called SIP protocol (Session Initiation Protocol), an entity which cannot process a request on account of a loading state can so indicate by rejecting this request with a specific cause of failure and can, in this rejection message, indicate a lag to be complied with before sending a new requisition to this entity.
The IETF document RFC 3265 describes a mechanism for managing events, within the SIP framework, using the SUBSCRIBE and NOTIFY messages and allowing asynchronous notification of events. The implementation of such a service based on the SIP protocol involves the cooperation of two appliances forming end nodes connected to the network. These services comprise automatic user callback (on the release of a call line previously called), notification of the presence of a user, notification to a user of the posting of a message in a voice box of the user, etc.
This application is limited however to appliances forming end nodes and to specific cases.
However, the existing schemes present a certain number of drawbacks.
First of all, most of them are adapted for allowing the spacing of the communications between two particular appliances identified in advance, thus rendering the system rather unadaptable. Moreover, other solutions such as those implemented within the framework of the SIP protocol are solutions in which the congested appliance can only trigger the communication spacing procedure when it receives a request and is already congested. The generation of the failure message has the effect of increasing its loading.
Additionally, other solutions provide for the development of a particular protocol dedicated to flow management, thus requiring an additional application level in the appliances of the network.